1. Field of the Invention
This invention relates generally to a current perpendicular to plane random access memory (CPP-MRAM) cell formed as a magnetic tunneling junction (MTJ)) and using a spin transfer effect with enhanced spin torque.
2. Description of the Related Art
The conventional magnetic tunneling junction (MTJ) device is a form of ultra-high giant magnetoresistive (GMR) device in which the relative orientation of the magnetic moments of parallel, vertically separated, upper and lower magnetized layers controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers. When injected electrons pass through the upper layer they are spin polarized by interaction with the magnetic moment of that layer. The majority of the electrons emerge polarized in the direction of the magnetic moment of the upper layer, the minority being polarized opposite to that direction. The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of multi-state resistor, since different relative orientations (for example, parallel and antiparallel) of the magnetic moments will change the magnitude of a current passing through the device. In a common type of device configuration (spin filter), one of the magnetic layers has its magnetic moment fixed in direction (pinned) by exchange coupling to an antiferromagnetic layer, while the other magnetic layer has its magnetic moment free to move (the free layer). Thus, such a device is a two state resistor. The magnetic moment of the free layer is then made to switch its direction from being parallel to that of the pinned layer, whereupon the tunneling current is large, to being antiparallel to the pinned layer, whereupon the tunneling current is small. The switching of the free layer moment direction (writing) is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell.
FIG. 1 is a highly schematic drawing showing an overhead view of a conventional MRAM cell between orthogonal word and bit lines. The cell is drawn with a slightly elliptical horizontal cross-section because such a shape produces a magnetic anisotropy within the free layer that assists its magnetic moment in retaining a thermally stable fixed position after switching fields have been turned off. The fields produced by currents in each of the two lines are between about 30 to 60 Oersteds in magnitude. According to the diagram, the word line field will be along the hard axis of the cell, the bit line field will be along the easy axis.
The use of magnetic fields externally generated by current carrying lines to switch the magnetic moment directions becomes problematic as the size of the MRAM cells decreases and, along with their decrease, so does the width of the current carrying lines. The smaller width lines require greater currents to produce the necessary switching fields, greatly increasing power consumption.
For this reason, a new type of magnetic device, called a spin transfer device and described by Slonczewski, (U.S. Pat. No. 5,695,164), has been developed and seems to eliminate some of the problems associated with the excessive power consumption necessitated by external switching fields. The spin transfer device shares some of the operational features of the conventional MTJ cell described above, except that the switching of the free layer magnetic moment is produced by the spin polarized current itself. In this device, unpolarized conduction electrons passing through a first magnetic layer having its magnetic moment oriented in a given direction (such as the pinned layer) are preferentially polarized by their passage through that layer by a quantum mechanical exchange interaction with the polarized bound electrons in the layer. Such a polarization can occur to conduction electrons that reflect from the surface of the magnetized layer as well as to those that pass through it. When such a stream of polarized conduction electrons subsequently pass through a second magnetic layer whose polarization direction is not fixed in space (a free layer), the polarized conduction electrons exert a torque on the bound electrons in the magnetic layer which, if sufficient, can reverse the polarization of the bound electrons and, thereby, reverse the magnetic moment of the magnetic layer. If the magnetic moment of the layer is directed along its easy magnetic axis, the required torque is minimized and the moment reversal occurs most easily. The use of a current internal to the cell to cause the magnetic moment reversal requires much smaller currents than those required to produce an external magnetic field from adjacent current carrying lines to produce the moment switching. Recent experimental data (W. H. Rippard et al., Phys. Rev. Lett., 92, (2004)) confirm magnetic moment transfer as a source of magnetic excitation and, subsequently, magnetic moment switching. These experiments confirm earlier theoretical predictions (J. C. Slonczewski, J. Magn. Mater., 159 (1996) LI, and J. Z. Sun, Phys. Rev. B., Vol. 62 (2000) 570). These latter papers show that the net torque, Γ, on the magnetization of a free magnetic layer produced by spin-transfer from a spin-polarized DC current is proportional to:Γ=snmx(nsxnm),  (1)Where s is the spin-angular momentum deposition rate, ns is a unit vector whose direction is that of the initial spin direction of the current and nm is a unit vector whose direction is that of the free layer magnetization and x represents a vector cross product. According to the equation, the torque is maximum when ns is orthogonal to nm.
Huai et al. (U.S. Pat. No. 6,714,444) describes a device utilizing spin transfer which is schematically illustrated in FIG. 2. This storage device consists of an underlayer (1), a first antiferromagnetic pinning layer (2), a first pinned ferromagnetic layer (3), a first non-magnetic spacer layer (4), a pinned ferromagnetic reference layer (5), a non-conducting tunneling barrier layer (6), a ferromagnetic free layer (7) (the storage layer), a non-magnetic layer (8), a pinned magnetic drive layer (9), a second non-magnetic spacing layer (10), a second pinned layer (11) and a second antiferromagnetic pinning layer (12) that pins the second pinned layer. Arrows drawn in the various layers, (200), (20), (30), (40) and (400), in the order of the layers discussed above, are exemplary magnetic moment directions. The double-headed arrow (30) in layer (7) indicates that this layer is free to have its magnetic moment directed in either of two directions.
Referring again to FIG. 2 it is noted that when the current is directed from bottom to top (layer (1) to layer (12)), conduction electrons are moving from top to bottom and will first pass through drive layer (9) before entering free layer (7). Therefore, the free layer magnetization would be switched to the direction of the drive layer's magnetization by the spin-transfer torque when the current density is larger than a critical value. Conversely, if the current is directed from top to bottom, the free layer magnetization would be switched to the direction of the pinned reference layer (5), since the conduction electrons have passed through that layer before entering the free layer.
In a typical prior art embodiment of the device shown in FIG. 2, the tunnel barrier layer (6) is a layer of AlOx (aluminum oxide), while non-magnetic layer (8) is a thin layer of Cu. The portion of the structure in FIG. 1 from layer (7) to (12) inclusive, is a CPP spin valve structure. The portion of the structure from (2) to (7) inclusive is a CPP magnetic tunneling (MTJ) structure in a spin filter configuration. Conduction electrons therefore, act on the free layer both by transmission (of the majority polarized electrons) and reflection (of the minority oppositely polarized electrons). Therefore, two spin torques would act on the free layer at the same time to minimize the required spin-transfer current.
The use of the spin transfer effect is also to be found in other prior art examples. Redon et al. (U.S. Pat. No. 6,532,164) discloses magnetic switching using spin transfer. Huai et al. (U.S. Pat. No. 7,126,202) discloses a spin transfer device that includes a thermal stabilization layer to maintain switched magnetic moment directions free from the thermal perturbations that can change the switched directions in a cell of small size.
The design of FIG. 1 has certain disadvantages if the tunneling barrier layer (6) is formed of MgO and the free layer (7) is CoFeB, which give very high values of DR/R (the magnetoresistance ratio, where DR is the difference between the maximum and minimum resistances of the device and R is the minimum resistance). A problem arises if the non-magnetic layer (8) is then formed of Cu, because such a layer deposited directly on top of the CoFeB free layer (7) would degrade DR/R as well as the spin momentum deposition rate s, (see equ. (1) above).
The use of CoFeB in conventional MRAM devices is well known in the prior art. Deak (U.S. Pat. No. 7,083,988) disclose a free layer comprising amorphous CoFeB and also amorphous CoFeB with a thin layer of Ta formed thereon. Slaughter et al. (U.S. Pat. No. 7,067,331) discloses free layers formed as laminations of CoFeB with Ru or Rh between the layers.
Since a low critical spin transfer current also requires a low magnetic moment of the free layer (7), which yields a lower energy barrier to the flipping of the magnetization direction, thermal instability (fluctuations in magnetization caused by thermal energy transfers) becomes a problem when the size of the MRAM cell becomes very small and domain structures do not provide the necessary stabilization. A conventional single ferromagnetic free layer patterned to a nanometer-scale shape appears to be unable to meet the required thermal stability condition, which is: E/kB T>40, where E is the energy barrier for magnetization flipping, T is the ambient temperature and kB is the Boltzmann constant. A synthetic ferromagnetic free layer is expected to satisfy this condition since such a structure consists of two ferromagnetic layers separated by a non-magnetic spacer layer, such as a layer of Ru, and the magnetization directions of the two layers are antiparallel, so that the resulting layer has a magnetic moment that is substantially zero, thereby presenting a substantially zero demagnetization field which helps the structure to withstand thermal fluctuations.
The present invention will describe a spin transfer MRAM device in which a new form of free layer will address the problems cited above.